During metastasis, carcinoma cells acquire the ability to invade extracellular matrix (ECM) to invade and intravasate. Although proteolytic activity is associated with increased metastasis and poor clinical outcome, the molecular triggers for matrix degradation in tumor cells are largely unknown. Our previous study demonstrated that the Twist1 transcription factor, a key regulator of early embryonic morphogenesis and a potent inducer of Epithelial-Mesenchymal Transition (EMT), plays an essential role in tumor metastasis. Our recent study found that a key role for Twist1 in metastasis is to promote matrix degradation and tumor invasion. Specifically, Twist1 induces the formation of invadopodia to perform local ECM degradation during tumor metastasis. Induction of invadopodia is essential for Twist1 to promote breast tumor metastasis in vivo and predicts poor survival in breast cancer patients. However, how Twist1 promotes invadopodia assembly and function are currently not well understood. Based on our preliminary data, we hypothesize that Twist1 induces the expression of an important membrane-associated protease that is essential for invadopodia-mediated ECM degradation and tumor metastasis. To test this hypothesis, our specific aims are as follows. Aim1: We aim to determine whether induction of this protease is required for Twist1-induced invadopodia formation and function and understand how Twist1 regulates its expression. Aim 2: We aim to determine how this protease regulates invadopodia assembly and function. Aim 3: We aim to determine whether co-expression of Twist1 and this protease is associated with distant metastasis and poor survival in human breast tumors and to test the in vivo importance of this protease in promoting tumor progression and metastasis in a mouse tumor metastasis model.